Nematodes, Biology tutorial


Nematode infections in humans comprise trichuriasis, ascariasis, hookworm, enterobiasis, filariasis, strongyloidiasis and trichinosis among others. The phylum Nematoda, as well termed as the roundworms, is the second biggest phylum in the animal kingdom, encompassing up to 500,000 species. Members of Nematoda are lengthened by bilaterally symmetric bodies which include an intestinal system and a big body cavity. Most of the roundworm species are free living in nature. Recent data have explained that around 60 species of roundworms parasitize humans. Intestinal roundworm infections comprise the biggest group of helminthic diseases in the humans.

General features of Nematodes:

Nematodes are cylindrical instead of flattened; therefore the common name roundworm. The body wall is comprised of an outer cuticle that consists of a non-cellular, chemically complex structure, a thin hypodermis and musculature. The cuticle in certain species consists of longitudinal ridges termed as alae. The bursa, a flap-like extension of the cuticle on the posterior end of certain species of male nematodes, is employed to grasp the female all through copulation.

The cellular hypodermis bulges to the body cavity or pseudocoelom to form four longitudinal cords; a dorsal, a ventral and two lateral cords which might be noticed on the surface as lateral lines. Nuclei of the hypodermis are positioned in the area of the cords. The somatic musculature lying under the hypodermis is a single layer of smooth muscle cells. If viewed in cross-section, this layer can be observed to be separated into four zones by the hypodermal cords. The musculature is innervated by extensions of muscle cells to the nerve trunks running anteriorly and posteriorly from ganglion cells which ring the mid-part of the esophagus. 

The space among the muscle layer and viscera is the pseudocoelom that lacks a mesothelium lining. This cavity includes fluid and 2-6 fixed cells (that is, celomocytes) which are generally related with the longitudinal cords. The function of such cells is unknown. 

The alimentary canal of roundworms is complete having both mouth and anus. The mouth is surrounded by lips bearing sensory papillae (that is, bristles). The oesophagus, a conspicuous attribute of nematodes, is a muscular structure which pumps food into the intestine; it distinct in shape in different species.  

The intestine is a tubular structure comprised of a single layer of columnar cells possessing prominent microvilli on the luminal surface.

The excretory system of a few nematodes comprises of an excretory gland and a pore positioned ventrally in the mid-esophageal area. In other nematodes this structure is drawn into extensions which give rise to the more complicated tubular excretory system that is generally H-shaped, having two anterior limbs and two posterior limbs positioned in the lateral cords. The gland cells and tubes are thought to serve as absorptive bodies, collecting wastes from the pseudocoelom, and to function in the Osmoregulation. 

Nematodes are generally bisexual. Males are generally smaller than females, encompass a curved posterior end and have (in some species) copulatory structures like spicules (generally two), a bursa or both. The males encompass one or (in a few cases) two testes that lie at the free end of a convoluted or recurved tube leading into a seminal vesicle and ultimately into the cloaca.

Basic life-cycle of the main groups of Nematodes:

The life cycles of the parasitic species differ considerably, as would be anticipated from such a big and diverse group. There are though a number of common features. First of all, the parasite experiences a sequence of moults via larval phases (designated L1 to the adult L5 form). Secondly, in most (however not all) nematodes it is the L3 larvae which is the infective form, significant exceptions to this being the Ascarids, like Ascaris lumbricoides and the pinworms, where it is either the L1 larvae or eggs having L1 or L2 larvae that are infective. Thirdly the L3 form onwards in all the species experiences a migration in the body of the definitive host as it matures to the adult parasite, generally via the bloodstream or lymphatic system to the heart, lungs, trachea and then to the intestine. At last, in most cases the parasite leaves the definitive host as thin walled eggs in the faeces, significant exceptions being the viviparous filarial worms (where  L1 larvae infect intermediate hosts, generally in the blood meals of the biting arthropods), Strongyloides stercoralis, (where the  L1 larvae are found in the faeces), and the viviparous Trichinela spiralis, where the larvae don't leave the body as such, however build up to the L3 phase which then encysts in the muscles, infection being by ingestion of undercooked contaminated meat. Infection of the definitive host might be by a diversity of routes, like the oral route, where eggs are accidentally ingested; as well numerous filarial worms are infective through the bite of flies, as formerly explained, and the L3 larvae of numerous like the hookworms and other related nematodes are directly invasive.

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